A large number of studies have been conducted on polymers formed from cage-type silsesquioxanes or their derivatives. These polymers are expected to have excellent properties in respect to heat resistance, weather resistance, optical characteristics, dimensional stability, and the like. For example, a process for producing a copolymer that consists of incompletely condensed silsesquioxanes (of an incomplete octagonal structure wherein the space is not closed due to cleavage of at least one bond) joined together by siloxane bonds is disclosed in the non-patent document 1. The process comprises introducing an amine or the like to a silsesquioxane of an incomplete cage structure through the aid of an organometallic compound and then crosslinking the resulting product with an aromatic imide or phenyl ether. Further, a process for producing a copolymer through the reaction of silanol groups possessed by a silsesquioxane of an incomplete cage structure with an aminosilane or the like is disclosed in the non-patent document 2.
In particular, electronic materials and optical materials are in need of improvements in heat resistance, durability, and moldability, and of further improvements in transparency and weather resistance depending upon the parts in which the materials are used. However, the conventional silsesquioxane copolymers have an indistinct structure and lack stability or cage-type silsesquioxanes undergo gelation when they are grafted to the main chain as they form a crosslinking point. Therefore, it is difficult to obtain a structure fully furnished with the aforementioned characteristics. For this reason, there is a demand for copolymers of excellent moldability in which a cage-type silsesquioxane of excellent heat resistance, weather resistance, and optical characteristics constitutes the main chain and the position of bond is clearly defined. However, there have been known few examples of copolymers in which a cage-type silsesquioxane is incorporated in the main chain.
Processes of copolymerization for producing a variety of copolymers are reported in the following patent documents 1 and 2; the processes comprise hydrolyzing a silane compound containing a trifunctional hydrolyzable group in an organic solvent in the presence of an alkali metal hydroxide to synthesize a silsesquioxane of an incomplete cage structure containing Si—ONa as a reactive group and then reacting this silsesquioxane of an incomplete cage structure with a chlorosilane that has a functional group capable of serving the purpose. To the knowledge of the inventors of this invention, however, no reports have been published other than the aforementioned ones. Moreover, a matter of concern here is that the aforementioned processes might restrict the side chains possessed by the cage-type silsesquioxane and provide poor heat resistance due to lack of curability. That is, materials of excellent properties are difficult to produce freely with good reproducibility by these methods.
By the way, studies on cage-type siloxanes or derivatives thereof are conducted actively (for example, refer to the non-patent document 3). Hydrolyzable group-containing derivatives of cage-type siloxanes are particularly useful as the reactivity of a hydrolyzable group can be utilized to derive new siloxane compounds. For example, Feher and co-workers report that they obtained a cage-type siloxane containing a silanol group by hydrolyzing a chlorosilane followed by aging (refer to the non-patent document 4).
However, the process of Feher and co-workers is problematical in that the synthesis requires a long time, the amount of by-products is large, and the yield of the target product is low. That is, a process for synthesizing a cage-type siloxane compound containing a silanol group that starts from a hydrolyzable group-containing monosilane faces the following problem; both hydrolysis and condensation reactions need to be controlled and, in addition, the molecular weight changes with passage of time as the condensation reaction proceeds between silanol groups being formed as the silanol group itself is extremely unstable. Therefore, it is substantially impossible to control the introduction of the silanol group freely. Recently, as the aforementioned patent documents 1 and 2 and the following patent document 3 propose, a silane compound containing a trifunctional hydrolyzable group is hydrolyzed in an organic solvent in the presence of an alkali metal hydroxide to yield a precursor in which Si—ONa is introduced as a reactive group in place of a silanol group (Si—OH) and the precursor is used for the production of silsesquioxane derivatives.    Patent document 1: WO2002/094839 pamphlet    Patent document 2: WO2003/024870 pamphlet    Patent document 3: JP2004-123698 A    Non-patent document 1: Chem. Mater., 2003, 15, 264-268    Non-patent document 2: Macromolecules, 1993, 26, 2141-2142    Non-patent document 3: Chem. Rev., 1995, 95, 1409    Non-patent document 4: Organometallics, 1991, 10, 2526